Square-Planar and Octahedral Gyroscope-Like Metal Complexes Consisting of Dipolar Rotators Encased in Dibridgehead Di(triaryl)phosphine Stators: Syntheses, Structures, Dynamic Properties, and Reactivity

For a variety of purposes, it is of interest to embed metals in cagelike trans-spanning di­(triaryl)­phosphine ligands. Toward this end, a combination of P­(p-C6H4O­(CH2) m CHCH2)3 [3; m = 4 (a), 5 (b), 6 (c), and 7 (d)], [Rh­(COD)­(μ-Cl)]2, and CO gives square-planar trans-Rh­(CO)­(Cl)­[P­(p-C6H4O­(CH2) m CHCH2)3]2 (4a–4d). Reactions of 4b–4d with Grubbs’ catalyst (first generation) and then H2 (catalyst PtO2) yield the title compounds trans-Rh­(CO)­(Cl)­[P­(p-C6H4O­(CH2) n O-p-C6H4)3P] (n = 2m + 2, 6b–6d; 26–41% from 4b–4d). Two are crystallographically characterized. The Cl–Rh–CO moieties rapidly rotate on the NMR time scale at −120 °C, per the ample clearance provided by the (CH2) n segments. Steric interactions with the PC6H4O linkages are analyzed. LiCCAr displaces the chloride ligand from 6b to give RhCCAr adducts (Ar = C6H5/p-C6H4CH3, 7b/8b). The ArCC–Rh–CO rotator of 7b rapidly rotates on the NMR time scale (−70 °C), but with 8b, the longer p-CH3C6H4CC group is confined between two (CH2)12 bridges, even at 120 °C. Reactions of Re­(CO)5(X) and 3c (140 °C) give octahedral mer,trans-Re­(CO)3(X)­[P­(p-C6H4O­(CH2)6CHCH2)3]2 (X = Cl/Br), and metathesis/hydrogenation sequences yield mer,trans-Re­(CO)3(X)­[P­(p-C6H4O­(CH2)14O-p-C6H4)3P]. Reactions of 6c and 6d and excess PMe3 give the free diphosphines P­(p-C6H4O­(CH2) n O-p-C6H4)3P (14c and 14d, 83–75%). The addition of 14d to [Rh­(CO)2(μ-Cl)]2 reconstitutes 6d (87%). Both in,in and out,out isomers of 14c and 14d are possible, but low-temperature NMR spectra show one set of signals, consistent with rapid homeomorphic isomerizations that turn the molecules inside out. Thermolyses (C6D5Br, 140 °C) effect phosphorus inversion to give in,out isomers.